This invention relates to a fault location device and method, useful for determining the position of a fault in a cable.
Buried underground cables and pipes (often referred to as xe2x80x9cservicesxe2x80x9d) sometimes develop faults and are not always clearly marked on plans, especially if they were not recently installed.
Precise fault location is desirable in the interests of reducing the amount of digging required to repair the cable. Underground cables can develop a number of different faults. Some examples are:
open circuit;
short circuit to another conductor;
short circuit to the shield; and
short circuit to the ground, also known as a sheath fault because the cable sheath has been breached.
A fault of primary interest is the sheath fault. One common method of detecting the location of this is to use a ground probe to search for maximum signal strength. This is effective because the ground is a volume conductor and the fault current is denser near the fault than it is as one moves further away. The maximum current point will be nearest the fault. One drawback of this method is that it is not possible to tell the direction to the fault and one must hunt to find whether one is moving closer to the fault or further from it. It is possible to address this limitation by various methods.
One method employs a large DC voltage to produce a deflection on a DC voltmeter. The direction of the deflection shows the direction to the fault, as the current flow is always away from the fault. The disadvantages of this method are that there are often DC current offsets in the soil which can give false readings and most cable connected equipment may not be tolerant of high DC voltage levels.
Another method is to use a very large voltage pulse to produce a deflection on a meter. The direction of the deflection shows the direction to the fault. The disadvantages if this method are that it is unreliable in wet soil, and the voltage pulse can damage the cable insulation or connected devices. The advantage over the DC voltage method is that it reduces problems due to local DC or very low frequency AC currents in the soil.
A further method involves adding a higher frequency carrier signal which is picked up by an antenna. This is processed and compared to the signal in the ground to determine the polarity of the signal in the ground. This method has the advantage that the ground signal can be a continuous AC signal. The drawback is that one must keep the receiving device locked to the cable frequency, and higher frequencies have a lower range.
Another method uses multiple harmonically related frequencies simultaneously to try and calculate the direction to the fault. The phase relationships of the signals are compared and the sign of the deviation between them is used to determine the direction to the fault. The sign of the phase deviation is equal to the polarity of the signal and hence the direction can be calculated. The advantages of this method are that it lessens the problems with DC signals, and also overcomes the need to keep the receiver locked to an external signal on the cable. The disadvantage is that errors in signal phase caused by noise and mains switching transients can produce incorrect results, effectively pointing the user of the device in the wrong direction.
A method of sheath fault location using a transmitter and a ground probe based receiver illustrated in FIG. 1.
A signal is placed on the cable 14 at a convenient access point by use of a generator 20. The return path for the signal is ground. The actual conductor used is the one with the sheath fault on it, otherwise there would be no return current. The fault is represented by an impedance 22 to ground and located at location 23 along the cable. In practice, the ground return path is complex and depends on the type of soil, moisture content, depth of cable and the presence of other buried conductors such as metal water pipes.
A ground probe 24, represented as a volt meter, is used to measure the voltage potential in the ground to determine the direction from the ground probe to the fault. This has two ground probe elements 24a, 24b which are positioned in spaced relationship in the ground, the probe if necessary being moved to various successive ground locations at which the probes are entered into the ground, and meter readings taken at the voltmeter.
Near to the fault, the ground currents branch out from the fault. Because of this, the ground probe can correctly identify the direction to the fault from either side of the fault. Directly over the fault, there is no signal at all and it is by determining the location of the probe at which this result ensues that the location 23 of the fault is determined. This is illustrated in FIG. 2 where the currents branching out from cable 14 at the fault are illustrated diagrammatically by arrows 26, and the polarities of detected signal at the voltmeter when positioned along the length of the cable, but to opposite sides of the fault is shown as being relatively reversed. There is no detected signal when the voltmeter is positioned adjacent the fault, and this is how the fault is located.
To reduce losses due to cable capacitance, lower frequencies are preferred. But frequencies in the normal operating range of the cable may cause cross talk and interference to other cables. As a result, frequencies below 300 Hz or above 3.4 KHz may be preferred. Frequencies below 300 Hz are however close to the harmonics and fundamentals of power frequency transmission equipment. As a result, signals well below 50/60 Hz may be most preferable.
There are three basic methods that might be used to show the direction to fault:
DC shift;
cable carrier and/or locked carrier reference; and
phase deviation.
The DC shift method involves either placing a large DC voltage on the cable or using a large pseudo impulse. The latter is preferable because it is less susceptible to local DC and low frequency AC currents, but both methods suffer from limited range in the wet and the possibility of damaging the cable.
The method cable carrier and/or locked carrier reference involves locking an on-board reference to the transmitter. This can be most easily achieved by sending a carrier signal down the same cable and picking it up with an antenna. Alternatively, a radio based carrier system could be used. Another method is to lock the receiver to the transmitter and hold the lock using a very low drift oscillator. In practice, a low drift oscillator locked to a cable borne signal may be more easily achieved. The disadvantage is that cable borne signal must be a high enough frequency to be readily picked up by a compact antenna and this normally brings it into or above the voice band. Signals in the voice band are not preferred by telecommunications carriers and higher frequencies are harder to keep phase aligned due to capacitive effects in the cable.
The phase deviation method involves using more than one frequency and measuring the direction of phase deviation between the two signals. If the direction of deviation is one polarity, then the fault lies in one direction, otherwise it lies in the other. These methods suffer from the fact that phase distortion or noise can cause erroneous results, even reversing the direction. Lower frequencies are preferred to improve range and reduce phase distortion due to capacitive effects, but because of the close proximity to mains power frequencies and their harmonics, substantial filtering is required. Filters are difficult to make phase shift free, and high Q band pass filters can ring in the presence of noise and switching transients, giving rise to false detections and incorrect direction results. At the very least, careful phase alignment is required.
In all of the above cases, it is difficult to provide high confidence that the user will correctly interpret the results and know when to ignore spurious readings.
The DC shift method involving high voltage pseudo impulses requires the operator to ignore slowly drifting meter movements, and to recognise a characteristic flicker due to the voltage spike. Audible feedback from the transmitter also helps for nearby faults. Unfortunately once the signal level drops, it is difficult for the operator to distinguish the random noise from the signal which is then likely to be minuscule.
The cable carrier systems and phase deviation systems have a similar difficulty. Once the signal level falls, the meter indicator can move sporadically in either direction, and it is hard for the user to objectively interpret the result.
Some systems attempt to also give the user an idea of the signal level, but this can also be misleading. A weak signal in a quiet area may be much more usable than a strong signal in a very noisy area such as near a mains power substation or railway line with track circuits energised to detect passing trains. Systems which ignore signals once the level falls sacrifice range.
The invention provides a device for determining location of a fault in an underground cable causing an earth leakage path from an internal conductor to earth at the location of the fault whereby, when signal is applied to the conductor, earth leakage signal flows between the earth and conductor at the location of the fault, wherein the applied signal is a multi-frequency signal having at least two frequency components, the device having probe means positionable to receive the earth leakage signal, and means for rectifying a first component of the earth leakage signal corresponding to one said frequency component of said applied signal, multiplying the rectified first component of the earth leakage signal with a second earth leakage signal corresponding to another frequency component of the applied signal and, from the result of said multiplication, determining the direction from the device to the fault.
Preferably, the device incorporates means for detecting and removing artifacts due to external interference such as switching transients.
Preferably, the device incorporates means for determining a confidence indication, indicating a degree of reliability of said result. The last-mentioned means may operate to determine said confidence indication by applying selected criteria to said result, such as the ratio between maximum positive and negative excursions thereof, and/or the signal-to-noise ratio of the detected signal and/or the absolute signal strength of the detected signal, and/or said signal components.
The invention also provides a method for determining location of a fault in an underground cable, wherein signal is applied to the cable to cause generation of an earth leakage signal from an internal conductor of the cable to earth, at the location of the fault, the applied signal being a multi-frequency signal having at least two frequency components, receiving the earth leakage signal, and rectifying a first component of the earth leakage signal corresponding to one frequency component of said applied signal, multiplying the rectified first component of the earth leakage signal with a second frequency component of the earth leakage signal corresponding to another said frequency component of the applied signal and, from the result of said multiplication, determining the direction from the device to the fault.
Some or all of signal processing may be effected digitally, under control of suitable software. The invention also provides a method for determining location of a fault in an underground cable, wherein signal is applied to the cable to cause generation of an earth leakage signal from an internal conductor of the cable to earth, at the location of the fault, the applied signal being a multi-frequency signal having at least two frequency components, receiving the earth leakage signal, and rectifying a first component of the earth leakage signal corresponding to one frequency component of said applied signal and multiplying the rectified first component of the earth leakage signal with a second frequency component of the earth leakage signal corresponding to another said frequency component of the applied signal, such that the result of said multiplying then represents the direction from the device to the fault. The multiplication may be effected as an array multiplication of sets of time-spaced samples of the first and second components of the earth leakage signal.